The invention relates to an anti-locking brake system for a land vehicle, which is set up in particular for traction control and vehicle dynamics control in such a way that a pump for actuating a wheel brake removes brake fluid in a controlled manner from a brake pressure generating unit and supplies it to the wheel brake.
From DE 40 35 527 A1 it is known to provide a 2-setting/2-position valve between the input side of the pump and the brake pressure generating unit. Said valve arrangement has only one actuation and/or connection setting. In said case, the flow cross section of said one actuation and/or connection setting has to be very large so that, upon entry into the directional stability control system, a rapid pressure build-up in the wheel brake is achieved via the pump, especially in order at low temperatures to achieve a good intake performance (high pumping capacity) of the pump. A large flow cross section however entails the drawbacks of loud hydraulic flow noises and severe pedal reactions, which is perceived as intrusive and far from comfortable.
Such a valve arrangement is known as a so-called xe2x80x9cnormal-closedxe2x80x9d (NC) valve. In the non-actuated state, i.e. the basic or normal position, the flow connection between the fluid connections of the valve arrangement is blocked, whereas in the actuated state the flow connection between the fluid connections is established. So that in the non-actuated state the flow connection between the fluid connections is blocked, a valve element under the action of a spring arrangement is biased so as to come sealingly into abutment with a sealing seat. In order in the actuated state to establish the flow connection between the fluid connections, it is necessary for the actuating device to summon up a force which is capable of surmounting at least the bias force of the spring arrangement so that the valve element lifts off the sealing seat.
However, in most applications it is not enough for the actuating device to summon up a force which is only slightly greater than the bias force of the spring arrangement because during operation different fluid pressures arise at the fluid connections of the valve arrangement, so that the valve element is additionally acted upon by a pressure differential force which, depending on the effective direction, is in the opposite direction to the force summoned up by the actuating device. The actuating device accordingly has to be designed in such a way that the actuating force is greater than the sum of the bias force of the spring arrangement and the maximum anticipated pressure differential force in order to guarantee reliable functioning of the valve arrangement. One drawback of this is that the actuating device has to be over dimensioned for many situations, particularly when, as in the majority of applications, in the actuated state of the valve arrangement a large flow cross section is required to prevent a throttling effect of the valve arrangement. A large flow cross section however additionally entails a very high pressure differential force, as a consequence of which a high actuating force has to be summoned up. The result is high costs and a high outlay when designing the valve arrangement. The size of the valve arrangement is also relatively large as a result.
In such a brake system which is set up for traction control and vehicle dynamics control it should be possible to generate, independently of a brake pedal actuation, brake pressure in the wheel brake/s to prevent by active brake engagement slipping of the driven wheels or skidding of the vehicle, especially when driving through curves. Such a brake systems is in a known manner bassed on a conventional brake system already adapted for anti lock control.
To be able to use the pump anyway provided in the anti lock control for returning brake fluid also for generating brake pressure in traction control and vehicle dynamics control, it is known from DE 38 32 023 C2 to additionally use a 3-setting/2-position valve which in a non-enabled condition, for a normal braking or anti lock control, establishes connection between the master cylinder and the output side of the pump and severs the connection to the input side of the pump. In the enabled condition the 3-setting/2-position valve severs for traction control the connection between master brake cylinder and output side of the pump and establishes the connection to the input side of the pump so that for generating brake pressure the pump can on the input side suck brake fluid from the master brake cylinder and at the output side feed it into the wheel brake/s. During traction control the brake pressure modulation is effected in a known manner by means of the valves provided for the anti-lock control.
There is the problem that brake pressure modulation results in considerably varying pressure levels between the output side of the pump and the wheel brake/s which causes intrusive hydraulic flow noises.
According to the above discussed DE 40 35 527 A1, now a valve combination is used which comprises two 2-connection/2-position valves instead of the additional 3-setting/2-position valve known from DE 38 32 023 C2, and which in an enabled condition severs both the connection between master cylinder and output side of the pump and the connection to the input side of the pump for the traction control.
Since the valve positions known from DE 38 32 023 C2 already allow pressure build-up and pressure reduction, this further valve position allows also to maintain the pressure as the input side of the pump is blocked so that no brake fluid can be taken. This is to make it possible to perform brake modulation during traction control exclusively via the two 2-setting/2-position valves, i.e. without switching the valves provided for the anti-lock control. Hydraulic flow noises as mentioned above are thus reduced.
However, it is a drawback in this connection that a precise brake pressure modulation is not possible during traction control. Even if the input side of the pump is blocked in order to maintain the pressure, there is always a small fluid volume at the input side of the pump which effects a minor pressure build-up rather than the desired maintenance of the pressure which as a whole renders pressure modulation inaccurate. Though the accuracy of the pressure modulation described in DE 40 35 527 A1 may be sufficient for traction control, it cannot at all or only by tolerating a restricted efficiency of the brake system be transferred onto a traction control because when driving through curves, an active brake intervention requires a precise brake pressure modulation to compensate for the vehicle""s tendency to oversteer or understeer. Too rough a brake pressure modulation would be impossible first of all for reasons of the brake system""s safety.
An object of the invention is to provide an anti-locking brake system while avoiding the previously described drawbacks. The brake system according to the invention is to meet the high demands made on a vehicle dynamics control and has at the same time to be applicable for the traction control.
To achieve the object, according to the invention an anti-locking brake system is further developed in that during the operating phase xe2x80x9cmaintain pressurexe2x80x9d, contrary to the position of the electromagnetic valve described in DE 40 35 527 A1, the third electromagnetic valve is in the actuated position. Accordingly, connection between the output connection of the pump and the wheel brake is blocked and a small fluid volume at the input side of the pump can not, as is possible with the brake system described in DE 40 35 527, be fed to the wheel brake through the pump. Due to this actually simple change of the position of the electromagnetic valve, surprisingly accuracy of the pressure modulation is considerably improved. This high accuracy is of importance especially for the vehicle dynamics control.
In addition, according to the invention, the anti-lock brake system described in DE 40 35 527 A1 has been developed further particularly in that between the brake pressure generating unit and an input connection of the pump a valve arrangement is provided which is designed as a 2-connection/3-position valve and which in a non-actuated (normal) position blocks the connection between the brake pressure generating unit and the input connection of the pump, in a first actuating position establishes a secondary flow connection between the brake pressure generating unit and the input connection of the pump, and in a second actuating position establishes a primary flow connection between the brake pressure generating unit and the input connection of the pump, wherein the primary flow connection has a larger flow cross section than the secondary flow connection.
All of the drawbacks of the previously described known arrangement (DE 40 35 527 A1) may be avoided by the solution according to the invention, which additionally provides a further actuating position with a reduced flow cross section. For, by said means, the volumetric flow from the brake pressure generating unit to the pump may be precisely metered. This applies particularly when the volumetric flow is modulated by a purposeful temporary switch between the normal position (1.1) and the first actuating position (1.2). Thus, during regulation of the brake pressure in the wheel brake the pressure level at the output side of the pump may be adapted almost to the pressure level in the wheel brake, with the added result that hydraulic noises are reduced.
A further advantage of the anti-locking brake system according to the invention is that, before the primary flow connection is established, a secondary flow connection is first established. Because of the secondary flow connection, in the situation where different fluid pressures at the fluid connections additionally exert a pressure differential force upon the valve element a pressure-equalizing process between the fluid connections is first initiated, thereby eliminating the pressure differential force so that during establishment of the primary flow connection the pressure differential force is no longer effective and may no longer have disadvantageous consequences. Thus, only a relatively low actuating force is required, with the result that the actuating device may be of a low-cost, simple design which takes up little installation space. As a result, the flow cross section determined mainly by the primary flow connection may be relatively generous in order to rule out an undesirable throttling effect in the flow behaviour of the valve arrangement. As the secondary flow connection has only a small flow cross section compared to the primary flow connection, the pressure differential force to be surmounted during the establishment of the second flow connection is negligibly low.
In an advantageous development, during an operating phase xe2x80x9creduce pressurexe2x80x9d the first valve arrangement is in the non-actuated position and the first, second and third electromagnetic valves are in the respective actuated position. Instead of allowing the pressure to only flow into the brake pressure generating unit, as described in DE 40 35 527 A1, according to the invention brake fluid is fed into the store through the open second electromagnetic valve, and thus conveyed away from the wheel brake in a defined manner. At the store, a counter pressure generated by a spring opposes the flowing-off brake fluid, so that the pressure decreases at an exactly predetermined rate. During this, the thus actuatedxe2x80x94namely blockedxe2x80x94third electromagnetic valve prevents that brake fluid under pressure gets from the braking line at the output connection of the pump to the wheel brake which would involuntarily weaken the pressure reduction.
The anti-locking brake system is advantageously developed further in such a manner that the first valve arrangement establishes in a further actuated position a secondary flow connection between the brake pressure generating unit and the input connection of the pump, with the secondary flow connection having a smaller flow cross-section than the primary flow connection, and that during an operating phase xe2x80x9cbuild up further pressurexe2x80x9d the first valve arrangement is in the further actuated position while the first electromagnetic valve is in the actuated position and the second and third electromagnetic valves each are in the non-actuated position. The above described further advantages of the invention and consequently an exactly defined pressure build-up at the wheel brake can be attained in this manner.
An advantageous development of the invention provides that during an operating phase xe2x80x9cmaintain further pressurexe2x80x9d the third electromagnetic valve is in the non-actuated position while the first electromagnetic valve is in the actuated position and the first valve arrangement and the second electromagnetic valve each are in the non-actuated position. In the operating phase xe2x80x9cbuild up further pressurexe2x80x9d preceding the operating phase xe2x80x9cmaintain further pressurexe2x80x9d the third electromagnetic valve is closed and the valve arrangement was opened only in a throttled manner. Thus the pump has generated a strong suction at the input connection so that at the beginning of the operating phase xe2x80x9cmaintain further pressurexe2x80x9d no fluid volume is at its input connection that might be conveyed through the pump and involuntarily increase the pressure at the wheel brake. Thus, according to the invention, at the beginning of the operating phase xe2x80x9cmaintain further pressurexe2x80x9d actuation of the third electromagnetic valve is dispensed with to reduce switching noises.
The anti-locking brake system may advantageously be developed further in that during an operating phase xe2x80x9creduce further pressurexe2x80x9d the third electromagnetic valve is in the non-actuated position while the first and second electromagnetic valves are in the actuated position each and the first valve arrangement is in the non-actuated position. Contrary to the above described operating phase xe2x80x9creduce pressurexe2x80x9d in which the third electromagnetic valve is actuatedxe2x80x94namely blockedxe2x80x94during the operating phase xe2x80x9creduce further pressurexe2x80x9d according to the invention the third electromagnetic valve may remain non-actuated. This is possible since a pressure compensation has taken place from the output connection of the pump through the opened third electromagnetic valve up to the wheel brake already during an operating phase xe2x80x9cmaintain further pressurexe2x80x9d, and brake fluid flows exclusively into the store during opening of the second electromagnetic valve at the beginning of the operating phase xe2x80x9creduce further pressurexe2x80x9d. This guarantees a controlled pressure reduction and at the same again reduces switching noises.
In a preferred form of construction of the invention, the valve arrangement comprises a first valve element, which is biased into the (normal) position by a spring arrangement, wherein a first actuating device is provided for bringing the valve element into the first actuating position, and a further valve element is provided, which is biased into the (normal) position by a further spring arrangement, and can be set by a further actuating device to the second actuating position, so that upon actuation of the valve arrangement initially only into the secondary flow connection and then the primary flow connection exist.
The actuating device and/or the further actuating device may preferably be electromagnetically and/or hydraulically controlled. Thus, in particular, the actuating device, which actuates the valve element to establish the primary flow connection, may be electromagnetically controlled and the further actuating device, which actuates the further valve element to establish the secondary flow connection, may be hydraulically controlled. In said case, the further hydraulically actuated actuating device may be controlled by means of the pressure difference existing between the fluid connections in order to establish the secondary flow connection for the purpose of initiating the pressure-equalizing process between the fluid connections. When after elimination of the pressure differential force the primary flow connection is established by the electromagnetically actuated actuating device, the actuating force to be summoned up is likewise only relatively low. This has a particularly advantageous effect upon the design of the electromagnetic arrangement of the actuating device, because the fact that the electromagnetic force to be summoned up is relatively low on the one hand means an energy-saving and hence inexpensive current input and on the other hand enables a compact design of the components of the magnetic circuit, above all of the coil and the armature.
The valve arrangement illustrated in FIG. 5 in the non-actuated condition and in FIG. 6 in the actuated condition is especially suited to be used as a two-connection/three-position valve inserted between the brake pressure generating unit and the input side of the pump. For its use here, the valve arrangement is flow-powered only in such a manner that with a first flow for establishing the first operating position only the further valve member is actuated (secondary flow connection B), and that with a second flow for establishing the second operating position the valve member is still actuated further (primary flow connection A).
According to a preferred embodiment, both the valve element and the further valve element can be actuated by a common actuating device. In particular, because of the previously described advantages, said common actuating device may be electromagnetically controlled. Said actuating device may comprise a two-stage plunger, which in dependence upon the armature moving the actuating device in a first stage actuates only the further valve element in order initially to establish the second flow connection, and then in a second stage to actuate additionally or exclusively the valve element, which establishes the primary flow connection characterizing the flow behaviour of the valve arrangement. Furthermore, one actuating device is saved, thereby allowing components and costs to be saved, and also a compact style of construction of the valve arrangement is achieved by virtue of the common actuating device.
In a particularly preferred embodiment, the valve element is dome-shaped and the further valve element is spherical, wherein the further valve element is disposed inside the valve element, thereby allowing the valve arrangement to be designed in a particularly space-saving manner.
The bias force of the spring arrangement is advantageously greater than the bias force of the further spring arrangement. As a result, the actuating force for establishing the secondary flow connection, which depends on the bias force of the further spring arrangement, is particularly low, above all when there are different fluid pressures at the fluid connections. The further spring arrangement may moreover be supported via the spring arrangement in order to achieve a simple and space-saving design of the valve arrangement.
From the aspects of simplicity and saving installation space, it is advantageous when the primary flow connection is determined by a sealing seat formed on the housing of the valve arrangement and by the valve element. The same applies when the secondary flow connection is formed by a sealing seat formed on the valve element and by the further valve element.
As the balance of forces is determined, on the one hand, by the actuating force to be summoned up by the actuating device and, on the other hand, by the bias force to be summoned up by the spring arrangement and, occasionally, by the pressure differential force between the fluid connections of the valve arrangement, with a view to simplicity and saving installation space the spring arrangement may be supported against the housing of the valve arrangement. The same advantage exists when the further spring arrangement is supported via the valve element.
An advantageous alternative is when the further valve element is integrally connected to the actuating device, with the result that components of the valve arrangement may be saved or may be easier to manufacture. In said case, to make the valve arrangement even more compact, it may be provided that the further spring arrangement biases the actuating device so that the further valve element occupies its first position, wherein the further spring arrangement is supported against the housing of the valve arrangement.
In a particularly preferred manner the valve arrangement according to the invention is to be used in an anti-locking brake system, which is set up for traction control and vehicle dynamics control, and to said end comprises a pump, which removes brake fluid from a brake pressure generating unit in order to supply the brake fluid to a wheel brake, wherein the first fluid connection of the valve arrangement is connected to the brake pressure generating unit and the second fluid connection of the valve arrangement is connected to the input side of the pump. Use of the valve arrangement according to the invention offers the great advantage that, on the one hand, a relatively large flow cross section is provided from the brake pressure generating unit to the input side of the pump so that the pump, above all at low temperatures, i.e. when the brake fluid behaves in a viscous manner, may deliver a relatively high volumetric flow. On the other hand, only a relatively low actuating force has to be summoned up when the valve arrangement is pressurized at the brake pressure generating unit side, which is the case upon actuation of the brake pressure generating unit by the driver via the brake pedal or upon automatic actuation of the brake pressure generating unit for preloading the pump.